Heat Index Pace Adjustment Calculator

How it is calculated

The tool converts air temperature to °F and computes the heat index with the Rothfusz regression, which uses temperature and relative humidity (%). Below roughly 80 °F it uses a simple average formula instead. The heat index in °F is then looked up in a slowdown table: under 60 °F 0 %, then 1.5, 3, 5, 7, 12, 15, 18 and 20 % in 5 °F steps up to 100 °F, and 25 % at 100 °F and above. Adjusted pace is cool pace × (1 + slowdown / 100), time lost is the pace difference times distance, and adjusted time is adjusted pace times distance. Pace is entered as decimal minutes per km (5.5 means 5:30), distance in km, and temperature must lie between −30 and 60 °C.

Common mistakes

• Typing the pace as 5.30 for 5 minutes 30 seconds. The field is decimal minutes, so 5.30 is 5 minutes 18 seconds; use 5.5.
• Entering humidity as a fraction such as 0.7. It must be a percentage from 0 to 100.
• Entering the temperature in °F. The field is °C, and 86 would be rejected as out of range.
• Using a hot-weather training pace as the cool pace. The tool applies the slowdown to whatever you enter, so a pace already slowed by heat is slowed again.
• Treating the slowdown as exact. It comes from steps in a banding table (a heat index of 94 °F and 95 °F gives 18 % and 20 %), and shade, wind and acclimation are not included.